This invention relates to chucks for holding a workpiece in a machine such as a lathe.
A variety of chucks or gripping devices are known in the machine and tool industry for holding a workpiece or a toolbit. Many of these chucks have movable or sliding jaws which can be moved radially inwardly or outwardly in order to hold or grip the end of a workpiece or a toolbit. These jaws are sometimes moved by means of a special tool or key that is turned manually.
One common form of chuck used with a lathe is called a scroll chuck which can be of the self-centering type. The number of jaws can vary from as few as two to as many as six or more but most commonly these chucks have three jaws that can be moved radially inwardly or outwardly by means of an annular scroll member that is turned about a central axis in order to maneuver the jaws. Scroll chucks have several advantages including versatility, good gripping force and ease of use. Often chucks of this type have a wide range of jaw movement enabling them to hold the ends of both small and large workpieces and they can be made to fit a variety of lathes.
However there is a serious disadvantage with scroll chucks made in the usual manner and this has restricted their use in some fields and for some applications. The difficulty arises when the scroll chuck has its jaws moved to the example, the individual jaws are arranged in a particular outermost position. In the case of a four jaw chuck, for order in order that they will advance properly in the slots of the jaw holder. The last jaw, jaw no. 4 is the final jaw to be engaged by the rib or scroll on the scroll member when the jaws are at their outermost position. If the jaw members are not checked prior to the startup of the lathe and the scroll member has not been properly advanced, there is a danger that one or more of the jaw members and in particular the fourth jaw member will not be properly engaged by the scroll member and will fly out of the chuck as soon as the lathe begins to operate. A jaw when thrown in this manner of course presents a considerable danger to any persons adjacent to the lathe.
The applicant is aware of only one previous attempt at solving this known problem with scroll chucks. In particular it is known to provide on at least the fourth jaw an inwardly projecting tab on the radially inner end of this jaw. The difficulty with this solution is that the tab seriously restricts the capacity of the scroll chuck. It also necessitates that the jaw device having the tab be disassembled in order for the jaws to be removed from the jaw holder.
It is an object of the present invention to provide an improved scroll chuck wherein the possibility of one or more jaws of the chuck becoming accidentally disengaged from the jaw holder is substantially reduced.
It is a further object of the invention to provide an improved scroll chuck provided with a jaw retention device on one of the jaws, which retention device extends into an elongate recess formed in the jaw holder and prevents the jaw device from disengaging from the jaw holder when the jaw devices have been moved to an outer position.